This invention generally relates to water-soluble and non-peptidic polymers, and methods of controlling the hydrolytic properties of such polymers.
Covalent attachment of the hydrophilic polymer poly(ethylene glycol), abbreviated PEG, also known as poly(ethylene oxide), abbreviated PEO, to molecules and surfaces is of considerable utility in biotechnology and medicine. In its most common form, PEG is a linear polymer terminated at each end with hydroxyl groups:
HOxe2x80x94CH2CH2Oxe2x80x94(CH2CH2O)nxe2x80x94CH2CH2xe2x80x94OH 
The above polymer, alpha-, omega-dihydroxylpoly(ethylene glycol), can be represented in brief form as HO-PEG-OH where it is understood that the -PEG- symbol represents the following structural unit:
xe2x80x94CH2CH2Oxe2x80x94(CH2CH2O)nxe2x80x94CH2CH2xe2x80x94
where n typically ranges from about 3 to about 4000.
PEG is commonly used as methoxy-PEG-OH, or mPEG in brief, in which one terminus is the relatively inert methoxy group, while the other terminus is a hydroxyl group that is subject to ready chemical modification. The structure of mPEG is given below.
CH3Oxe2x80x94(CH2CH2O)nxe2x80x94CH2CH2xe2x80x94OH 
Random or block copolymers of ethylene oxide and propylene oxide, shown below, are closely related to PEG in their chemistry, and they can be substituted for PEG in many of its applications.
HOxe2x80x94CH2CHRO(CH2CHRO)nCH2CHRxe2x80x94OH 
wherein each R is independently H or CH3.
To couple PEG to a molecule, such as a protein, it is often necessary to xe2x80x9cactivatexe2x80x9d the PEG by preparing a derivative of the PEG having a functional group at a terminus thereof. The functional group is chosen based on the type of available reactive group on the molecule that will be coupled to the PEG. For example, the functional group could be chosen to react with an amino group on a protein in order to form a PEG-protein conjugate.
PEG is a polymer having the properties of solubility in water and in many organic solvents, lack of toxicity, and lack of immunogenicity. One use of PEG is to covalently attach the polymer to insoluble molecules to make the resulting PEG-molecule xe2x80x9cconjugatexe2x80x9d soluble. For example, it has been shown that the water-insoluble drug paclitaxel, when coupled to PEG, becomes water-soluble. Greenwald, et al., J. Org. Chem., 60:331-336 (1995).
The prodrug approach, in which drugs are released by degradation of more complex molecules (prodrugs) under physiological conditions, is a powerful component of drug delivery. Prodrugs can, for example, be formed by bonding PEG to drugs using linkages which are degradable under physiological conditions. The lifetime of PEG prodrugs in vivo depends upon the type of functional group linking PEG to the drug. In general, ester linkages, formed by reaction of PEG carboxylic acids or activated PEG carboxylic acids with alcohol groups on the drug, hydrolyze under physiological conditions to release the drug, while amide and carbamate linkages, formed from amine groups on the drug, are stable and do not hydrolyze to release the free drug.
Use of certain activated esters of PEG, such as N-hydroxylsuccinimide esters, can be problematic because these esters are so reactive that hydrolysis of the ester takes place almost immediately in aqueous solution. It has been shown that hydrolytic delivery of drugs from PEG esters can be favorably controlled to a certain extent by controlling the number of linking methylene groups in a spacer between the terminal PEG oxygen and the carbonyl group of the attached carboxylic acid or carboxylic acid derivative. For example, Harris et al., in U.S. Pat. No. 5,672,662, describe PEG butanoic acid and PEG propanoic acid (shown below), and activated derivatives thereof, as alternatives to carboxymethyl PEG (also shown below) when less hydrolytic reactivity in the corresponding ester derivatives is desirable.
PEG-OCH2CH2CH2CO2H 
PEG butanoic acid
PEG-O-CH2CH2CO2H 
PEG propanoic acid
PEG-Oxe2x80x94CH2CO2H
carboxymethyl PEG
In aqueous buffers, hydrolysis of esters of these modified PEG acids can be controlled in a useful way by varying the number of xe2x80x94CH2-spacers between the carboxyl group and the PEG oxygen.
There remains a need in the art for further methods of controlling the hydrolytic degradation of activated polymer derivatives.
The invention provides a group of water-soluble and non-peptidic polymers having at least one terminal carboxylic acid or carboxylic acid derivative group. The acid or acid derivative group of the polymer is sterically hindered by the presence of an alkyl or aryl group on the carbon adjacent to the carbonyl group of the carboxylic acid (xcex1-carbon). The steric effect of the alkyl or aryl group enables greater control of the rate of hydrolytic degradation of polymer derivatives. For example, both activated carboxylic acid derivatives, such as succinimidyl esters, and biologically active polymer conjugates resulting from the coupling of the polymers of the invention to biologically active agents, such as small drug molecules, enzymes or proteins, are more hydrolytically stable due to the presence of the xcex1-carbon alkyl or aryl group.
The sterically hindered polymers of the invention comprise a water-soluble and non-peptidic polymer backbone having at least one terminus, the terminus being covalently bonded to the structure 
wherein:
L is the point of bonding to the terminus of the polymer backbone;
Q is O or S;
m is 0 to about 20;
Z is selected from the group consisting of alkyl, substituted alkyl, aryl and substituted aryl; and
X is a leaving group.
Examples of suitable water-soluble and non-peptidic polymer backbones include poly(alkylene glycol), poly(oxyethylated polyol), poly(olefinic alcohol), poly(vinylpyrrolidone), poly(hydroxypropylmethacrylamide), poly(xcex1-hydroxy acid), poly(vinyl alcohol), polyphosphazene, polyoxazoline, poly(N-acryloylmorpholine), and copolymers, terpolymers, and mixtures thereof. In one embodiment, the polymer backbone is poly(ethylene glycol) having an average molecular weight from about 200 Da to about 100,000 Da.
Examples of the Z moiety include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, t-butyl, and benzyl. In one embodiment, Z is a C1-C8 alkyl or substituted alkyl.
The leaving group, X, can be, for example, halogen, such as chlorine or bromine, N-succinimidyloxy, sulfo-N-succinimidyloxy, 1-benzotriazolyloxy, hydroxyl, 1-imidazolyl, and p-nitrophenyloxy.
The invention also includes biologically active conjugates of the polymers of the invention and biologically active agents and methods of making such conjugates.
By changing the length or size of the alkyl or aryl group used as the Z moiety, the polymers of the invention offer an increased ability to control and manipulate the hydrolytic stability of polymer derivatives prepared using the polymers. Better control of the rate of hydrolytic degradation enables the practitioner to tailor polymer constructs for specific end uses that require certain degradation properties.
The terms xe2x80x9cfunctional groupxe2x80x9d, xe2x80x9cactive moietyxe2x80x9d, xe2x80x9cactivating groupxe2x80x9d, xe2x80x9creactive sitexe2x80x9d, xe2x80x9cchemically reactive groupxe2x80x9d and xe2x80x9cchemically reactive moietyxe2x80x9d are used in the art and herein to refer to distinct, definable portions or units of a molecule. The terms are somewhat synonymous in the chemical arts and are used herein to indicate that the portions of molecules that perform some function or activity and are reactive with other molecules. The term xe2x80x9cactive,xe2x80x9d when used in conjunction with functional groups, is intended to include those functional groups that react readily with electrophilic or nucleophilic groups on other molecules, in contrast to those groups that require strong catalysts or highly impractical reaction conditions in order to react. For example, as would be understood in the art, the term xe2x80x9cactive esterxe2x80x9d would include those esters that react readily with nucleophilic groups such as amines. Typically, an active ester will react with an amine in aqueous medium in a matter of minutes, whereas certain esters, such as methyl or ethyl esters, require a strong catalyst in order to react with a nucleophilic group.
The term xe2x80x9clinkagexe2x80x9d or xe2x80x9clinkerxe2x80x9d is used herein to refer to groups or bonds that normally are formed as the result of a chemical reaction and typically are covalent linkages. Hydrolytically stable linkages means that the linkages are substantially stable in water and do not react with water at useful pHs, e.g., under physiological conditions for an extended period of time, perhaps even indefinitely. Hydrolytically unstable or degradable linkages means that the linkages are degradable in water or in aqueous solutions, including for example, blood. Enzymatically unstable or degradable linkages means that the linkage can be degraded by one or more enzymes. As understood in the art, PEG and related polymers may include degradable linkages in the polymer backbone or in the linker group between the polymer backbone and one or more of the terminal functional groups of the polymer molecule. For example, ester linkages formed by the reaction of PEG carboxylic acids or activated PEG carboxylic acids with alcohol groups on a biologically active agent generally hydrolyze under physiological conditions to release the agent. Other hydrolytically degradable linkages include carbonate linkages; imine linkages resulted from reaction of an amine and an aldehyde (see, e.g., Ouchi et al., Polymer Preprints, 38(1):582-3 (1997), which is incorporated herein by reference.); phosphate ester linkages formed by reacting an alcohol with a phosphate group; hydrozone linkages which are reaction product of a hydrazide and an aldehyde; acetal linkages that are the reaction product of an aldehyde and an alcohol; orthoester linkages that are the reaction product of a formate and an alcohol; peptide linkages formed by an amine group, e.g., at an end of a polymer such as PEG, and a carboxyl group of a peptide; and oligonucleotide linkages formed by a phosphoramidite group, e.g., at the end of a polymer, and a 5xe2x80x2 hydroxyl group of an oligonucleotide.
The term xe2x80x9cbiologically active moleculexe2x80x9d, xe2x80x9cbiologically active moietyxe2x80x9d or xe2x80x9cbiologically active agentxe2x80x9d when used herein means any substance which can affect any physical or biochemical properties of a biological organism, including but not limited to viruses, bacteria, fungi, plants, animals, and humans. In particular, as used herein, biologically active molecules include any substance intended for diagnosis, cure mitigation, treatment, or prevention of disease in humans or other animals, or to otherwise enhance physical or mental well-being of humans or animals. Examples of biologically active molecules include, but are not limited to, peptides, proteins, enzymes, small molecule drugs, dyes, lipids, nucleosides, oligonucleotides, cells, viruses, liposomes, microparticles and micelles. Classes of biologically active agents that are suitable for use with the invention include, but are not limited to, antibiotics, fungicides, anti-viral agents, anti-inflammatory agents, anti-tumor agents, cardiovascular agents, anti-anxiety agents, hormones, growth factors, steroidal agents, and the like.
The terms xe2x80x9calkyl,xe2x80x9d xe2x80x9calkene,xe2x80x9d and xe2x80x9calkoxyxe2x80x9d include straight chain and branched alkyl, alkene, and alkoxy, respectively. The term xe2x80x9clower alkylxe2x80x9d refers to C1-C6 alkyl. The term xe2x80x9calkoxyxe2x80x9d refers to oxygen substituted alkyl, for example, of the formulas xe2x80x94OR or xe2x80x94ROR1, wherein R and R1 are each independently selected alkyl. The terms xe2x80x9csubstituted alkylxe2x80x9d and xe2x80x9csubstituted alkenexe2x80x9d refer to alkyl and alkene, respectively, substituted with one or more non-interfering substituents, such as but not limited to, C3-C6 cycloalkyl, e.g., cyclopropyl, cyclobutyl, and the like; acetylene; cyano; alkoxy, e.g., methoxy, ethoxy, and the like; lower alkanoyloxy, e.g., acetoxy; hydroxy; carboxyl; amino; lower alkylamino, e.g., methylamino; ketone; halo, e.g. chloro or bromo; phenyl; substituted phenyl, and the like. The term xe2x80x9chalogenxe2x80x9d includes fluorine, chlorine, iodine and bromine.
xe2x80x9cArylxe2x80x9d means one or more aromatic rings, each of 5 or 6 carbon atoms. Multiple aryl rings may be fused, as in naphthyl or unfused, as in biphenyl. Aryl rings may also be fused or unfused with one or more cyclic hydrocarbon, heteroaryl, or heterocyclic rings.
xe2x80x9cSubstituted arylxe2x80x9d is aryl having one or more non-interfering groups as substituents.
xe2x80x9cNon-interfering substituentsxe2x80x9d are those groups that yield stable compounds. Suitable non-interfering substituents or radicals include, but are not limited to, halo, C1-C10 alkyl, C2-C10 alkenyl, C2-C10 alkynyl, C1-C10 alkoxy, C7-C12 aralkyl, C7-C12 alkaryl, C3-C10 cycloalkyl, C3-C10 cycloalkenyl, phenyl, substituted phenyl, toluoyl, xylenyl, biphenyl, C2-C12 alkoxyalkyl, C7-C12 alkoxyaryl, C7-C12 aryloxyalkyl, C6-C12 oxyaryl, C1-C6 alkylsulfinyl, C1-C10 alkylsulfonyl, xe2x80x94(CH2)mxe2x80x94Oxe2x80x94(C1-C10 alkyl) wherein m is from 1 to 8, aryl, substituted aryl, substituted alkoxy, fluoroalkyl, heterocyclic radical, substituted heterocyclic radical, nitroalkyl, xe2x80x94NO2, xe2x80x94CN, xe2x80x94NRC(O)-(C1-C10 alkyl), xe2x80x94C(O)xe2x80x94(C1-C10 alkyl), C2-C10 thioalkyl, xe2x80x94C(O)Oxe2x80x94(C1-C10 alkyl), xe2x80x94OH, xe2x80x94SO2, xe2x95x90S, xe2x80x94COOH, xe2x80x94NR2, carbonyl, xe2x80x94C(O)xe2x80x94(C1-C10 alkyl)-CF3, xe2x80x94C(O)xe2x80x94CF3, xe2x80x94C(O)NR2, xe2x80x94(C1-C10 alkyl)xe2x80x94Sxe2x80x94(C6-C12 aryl), xe2x80x94C(O)xe2x80x94(C6-C12 aryl), xe2x80x94(CH2)mxe2x80x94Oxe2x80x94(CH2)mxe2x80x94Oxe2x80x94(C1-C10 alkyl) wherein each m is from 1 to 8, xe2x80x94C(O)NR2, xe2x80x94C(S)NR2, xe2x80x94SO2NR2, xe2x80x94NRC(O)NR2, xe2x80x94NRC(S)NR2, salts thereof, and the like. Each R as used herein is H, alkyl or substituted alkyl, aryl or substituted aryl, aralkyl, or alkaryl.
The invention provides a sterically hindered polymer, comprising a water-soluble and non-peptidic polymer backbone having at least one terminus, the terminus being covalently bonded to the structure 
wherein:
L is the point of bonding to the terminus of the polymer backbone;
Q is O or S;
m is 0 to about 20;
Z is selected from the group consisting of alkyl, substituted alkyl, aryl and substituted aryl; and
X is a leaving group.
The polymer backbone of the water-soluble and non-peptidic polymer can be poly(ethylene glycol) (i.e. PEG). However, it should be understood that other related polymers are also suitable for use in the practice of this invention and that the use of the term PEG or poly(ethylene glycol) is intended to be inclusive and not exclusive in this respect. The term PEG includes poly(ethylene glycol) in any of its forms, including alkoxy PEG, difunctional PEG, multiarmed PEG, forked PEG, branched PEG, pendent PEG, or PEG with degradable linkages therein.
PEG is typically clear, colorless, odorless, soluble in water, stable to heat, inert to many chemical agents, does not hydrolyze or deteriorate, and is generally non-toxic. Poly(ethylene glycol) is considered to be biocompatible, which is to say that PEG is capable of coexistence with living tissues or organisms without causing harm. More specifically, PEG is substantially non-immunogenic, which is to say that PEG does not tend to produce an immune response in the body. When attached to a molecule having some desirable function in the body, such as a biologically active agent, the PEG tends to mask the agent and can reduce or eliminate any immune response so that an organism can tolerate the presence of the agent. PEG conjugates tend not to produce a substantial immune response or cause clotting or other undesirable effects. PEG having the formula xe2x80x94CH2CH2Oxe2x80x94(CH2CH2O)nxe2x80x94CH2CH2xe2x80x94, where n is from about 3 to about 4000, typically from about 3 to about 2000, is one useful polymer in the practice of the invention. PEG having a molecular weight of from about 200 Da to about 100,000 Da are particularly useful as the polymer backbone.
The polymer backbone can be linear or branched. Branched polymer backbones are generally known in the art. Typically, a branched polymer has a central branch core moiety and a plurality of linear polymer chains linked to the central branch core. PEG is commonly used in branched forms that can be prepared by addition of ethylene oxide to various polyols, such as glycerol, pentaerythritol and sorbitol. The central branch moiety can also be derived from several amino acids, such as lysine. The branched poly(ethylene glycol) can be represented in general form as R(xe2x80x94PEG-OH)m in which R represents the core moiety, such as glycerol or pentaerythritol, and m represents the number of arms. Multi-armed PEG molecules, such as those described in U.S. Pat. No. 5,932,462, which is incorporated by reference herein in its entirety, can also be used as the polymer backbone.
Branched PEG can also be in the form of a forked PEG represented by PEG(-YCHZ2)n, where Y is a linking group and Z is an activated terminal group linked to CH by a chain of atoms of defined length.
Yet another branched form, the pendant PEG, has reactive groups, such as carboxyl, along the PEG backbone rather than at the end of PEG chains.
In addition to these forms of PEG, the polymer can also be prepared with weak or degradable linkages in the backbone. For example, PEG can be prepared with ester linkages in the polymer backbone that are subject to hydrolysis. As shown below, this hydrolysis results in cleavage of the polymer into fragments of lower molecular weight: 
It is understood by those skilled in the art that the term poly(ethylene glycol) or PEG represents or includes all the above forms.
Many other polymers are also suitable for the invention. Polymer backbones that are non-peptidic and water-soluble, with from 2 to about 300 termini, are particularly useful in the invention. Examples of suitable polymers include, but are not limited to, other poly(alkylene glycols), such as poly(propylene glycol) (xe2x80x9cPPGxe2x80x9d), copolymers of ethylene glycol and propylene glycol and the like, poly(oxyethylated polyol), poly(olefinic alcohol), poly(vinylpyrrolidone), poly(hydroxypropylmethacrylamide), poly(xcex1-hydroxy acid), poly(vinyl alcohol), polyphosphazene, polyoxazoline, poly(N-acryloylmorpholine), such as described in U.S. Pat. No. 5,629,384, which is incorporated by reference herein in its entirety, and copolymers, terpolymers, and mixtures thereof Although the molecular weight of each chain of the polymer backbone can vary, it is typically in the range of from about 100 Da to about 100,000 Da, often from about 6,000 Da to about 80,000 Da.
Those of ordinary skill in the art will recognize that the foregoing list for substantially water soluble and non-peptidic polymer backbones is by no means exhaustive and is merely illustrative, and that all polymeric materials having the qualities described above are contemplated.
Examples of suitable alkyl and aryl groups for the Z moiety include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, t-butyl, and benzyl. In one embodiment, Z is a C1-C8 alkyl or substituted alkyl.
The optional CH2 spacer between the xcex1-carbon and the Q moiety can provide additional dampening effect on the rate of hydrolytic degradation of the molecule. In one embodiment, m is 1 to about 10.
The X moiety is a leaving group, meaning that it can be displaced by reaction of a nucleophile with the molecule containing X. In some cases, as when X is hydroxy, the group must be activated by reaction with a molecule such as N,Nxe2x80x2-dicyclohexylcarbodiimide (DCC) in order to make it an effective leaving group. Examples of suitable X moieties include halogen, such as chlorine and bromine, N-succinimidyloxy, sulfo-N-succinimidyloxy, 1-benzotriazolyloxy, hydroxyl, 1-imidazolyl, and p-nitrophenyloxy. In one aspect, the polymer has a terminal carboxylic acid group (i.e. X is hydroxyl).
In one embodiment, the polymer of the invention has the structure 
wherein:
POLY is a water-soluble and non-peptidic polymer backbone, such as PEG;
Rxe2x80x2 is a capping group; and
Q, m, Z and X are as defined above.
Rxe2x80x2 can be any suitable capping group known in the art for polymers of this type. For example, Rxe2x80x2 can be a relatively inert capping group, such as an alkoxy group (e.g. methoxy).
Alternatively, Rxe2x80x2 can be a functional group. Examples of suitable functional groups include hydroxyl, protected hydroxyl, active ester, such as N-hydroxysuccinimidyl esters and 1-benzotriazolyl esters, active carbonate, such as N-hydroxysuccinimidyl carbonates and 1-benzotriazolyl carbonates, acetal, aldehyde, aldehyde hydrates, alkenyl, acrylate, methacrylate, acrylamide, active sulfone, amine, protected amine, hydrazide, protected hydrazide, thiol, protected thiol, carboxylic acid, protected carboxylic acid, isocyanate, isothiocyanate, maleimide, vinylsulfone, dithiopyridine, vinylpyridine, iodoacetamide, epoxide, glyoxals, diones, mesylates, tosylates, and tresylate. The functional group is typically chosen for attachment to a functional group on a biologically active agent. As would be understood, the selected Rxe2x80x2 moiety should be compatible with the X group so that reaction with X does not occur.
As would be understood in the art, the term xe2x80x9cprotectedxe2x80x9d refers to the presence of a protecting group or moiety that prevents reaction of the chemically reactive functional group under certain reaction conditions. The protecting group will vary depending on the type of chemically reactive group being protected. For example, if the chemically reactive group is an amine or a hydrazide, the protecting group can be selected from the group of tert-butyloxycarbonyl (t-Boc) and 9-fluorenylmethoxycarbonyl (Fmoc). If the chemically reactive group is a thiol, the protecting group can be orthopyridyldisulfide. If the chemically reactive group is a carboxylic acid, such as butanoic or propionic acid, or a hydroxyl group, the protecting group can be benzyl or an alkyl group such as methyl or ethyl. Other protecting groups known in the art may also be used in the invention.
Specific examples of terminal functional groups in the literature include N-succinimidyl carbonate (see e.g., U.S. Pat. Nos. 5,281,698, 5,468,478), amine (see, e.g., Buckmann et al. Makromol. Chem. 182:1379 (1981), Zaplipsky et al. Eur. Polym. J. 19:1177 (1983)), hydrazide (See, e.g., Andresz et al. Makromol. Chem. 179:301 (1978)), succinimidyl propionate and succinimidyl butanoate (see, e.g., Olson et al. in Poly(ethylene glycol) Chemistry and Biological Applications, pp 170-181, Harris and Zaplipsky Eds., ACS, Washington, D.C., 1997; see also U.S. Pat. No. 5,672,662), succinimidyl succinate (See, e.g., Abuchowski et al. Cancer Biochem. Biophys. 7:175 (1984) and Joppich et al. Macrolol. Chem. 180:1381 (1979), succinimidyl ester (see, e.g., U.S. Pat. No. 4,670,417), benzotriazole carbonate (see, e.g., U.S. Pat. No. 5,650,234), glycidyl ether (see, e.g., Pitha et al. Eur. J Biochem. 94:11 (1979), Elling et al., Biotech. Appl. Biochem. 13:354 (1991), oxycarbonylimidazole (see, e.g., Beauchamp, et al., Anal. Biochem. 131:25 (1983), Tondelli et al. J. Controlled Release 1:251 (1985)), p-nitrophenyl carbonate (see, e.g., Veronese, et al., Appl. Biochem. Biotech., 11: 141 (1985); and Sartore et al., Appl. Biochem. Biotech., 27:45 (1991)), aldehyde (see, e.g., Harris et al. J. Polym. Sci. Chem. Ed. 22:341 (1984), U.S. Pat. No. 5,824,784, U.S. Pat. No. 5,252,714), maleimide (see, e.g., Goodson et al. Bio/Technology 8:343 (1990), Romani et al. in Chemistry of Peptides and Proteins 2:29 (1984)), and Kogan, Synthetic Comm. 22:2417 (1992)), orthopyridyl-disulfide (see, e.g., Woghiren, et al. Bioconj. Chem. 4:314 (1993)), acrylol (see, e.g., Sawhney et al., Macromolecules, 26:581 (1993)), vinylsulfone (see, e.g., U.S. Pat. No. 5,900,461). In addition, two molecules of the polymer of this invention can also be linked to the amino acid lysine to form a di-substituted lysine, which can then be further activated with N-hydroxysuccinimide to form an active N-succinimidyl moiety (see, e.g., U.S. Pat. No. 5,932,462). All of the above references are incorporated herein by reference.
Rxe2x80x2 can also have the structure -W-D, wherein W is a linker and D is a biologically active agent. Alternatively, the polymer structure can be a homobifunctional molecule such that Rxe2x80x2 is xe2x80x94Q(CH2)mCHZC(O)X, wherein Q, m, Z and X are as defined above.
An example of a multi-arm polymer of the invention is shown below: 
wherein:
POLY is a water-soluble and non-peptidic polymer backbone, such as PEG;
R is a central core molecule, such as glycerol or pentaerythritol;
q is an integer from 2 to about 300; and
Q, m, Z and X are as defined above.
Further examples of the polymers of the invention include polymers of the structure 
wherein:
PEG is poly(ethylene glycol); and
X, m and Z are as defined above.
The polymers of the invention, whether activated or not, can be purified from the reaction mixture. One method of purification involves precipitation from a solvent in which the polymers are essentially insoluble while the reactants are soluble. Suitable solvents include ethyl ether or isopropanol. Alternatively, the polymers may be purified using ion exchange, size exclusion, silica gel, or reverse phase chromatography.
In all the above embodiments, the presence of the xcex1-alkyl or xcex1-aryl group (Z) confers upon the polymer greater stability to hydrolysis due to the steric and electronic effect of the alkyl or aryl group. The steric effect may be increased by increasing the size of the alkyl or aryl group, as would be the case in replacing methyl with ethyl. In other words, as the number of carbon atoms in Z increases, the rate of hydrolysis decreases. As noted above, use of this steric effect may also be applied in combination with the electronic effect obtained by variation in the distance of the Q moiety from the carboxyl group (i.e. control of the value of m). By controlling both m and Z, the rate of hydrolysis can be regulated in a more flexible manner.
Since the enzyme catalyzed reactions that cause enzymatic degradation involve exact spatial fits between the enzyme active site and the polymer, steric effects can be very important in these reactions as well. The polymers of the invention can also be used to better regulate or control enzymatic degradation in addition to hydrolytic degradation.
When coupled to biologically active agents, the polymers of the invention will help regulate the rate of hydrolytic degradation of the resulting polymer conjugate. As an example, when the polymers of the invention are coupled with alcohols or thiols to form esters or thioesters respectively, the esters or thioesters are more stable to hydrolysis. Thus, a drug bearing an alcohol or thiol group may be derivatized with a polymer of the invention and the hydrolytic release of the drug from such esters or thiolesters can be controlled by choice of the xcex1-alkyl or xcex1-aryl group.
The invention provides a biologically active polymer conjugate comprising a water-soluble and non-peptidic polymer backbone having at least one terminus, the terminus being covalently bonded to the structure 
wherein:
L is the point of bonding to the terminus of the polymer backbone;
Q is O or S;
m is 0 to about 20;
Z is selected from the group consisting of alky, substituted alkyl, aryl and substituted aryl;
W is a linker; and
D is a biologically active agent.
The linker W is the residue of the functional group used to attach the biologically active agent to the polymer backbone. In one embodiment, W is O, S, or NH.
Examples of suitable biologically active agents include peptides, proteins, enzymes, small molecule drugs, dyes, lipids, nucleosides, oligonucleotides, cells, viruses, liposomes, microparticles and micelles.
The invention also includes a method of preparing biologically active conjugates of the polymers of the invention by reacting a polymer of Formula I with a biologically active agent.
The following examples are given to illustrate the invention, but should not be considered in limitation of the invention.